Selective catalytic reduction (SCR) catalysts may be utilized in the exhaust systems of engines (e.g., diesel engines or other lean-burn engines) to reduce nitrogen oxide (NOx) emissions. A reductant, such as urea, may be injected into the exhaust system upstream of the SCR catalyst, and together, the reductant and the SCR catalyst may chemically reduce NOx molecules to nitrogen and water, thereby limiting NOx emissions. However, if a component of the NOx emission control system, such as the SCR catalyst, becomes degraded, NOx emissions may increase. NOx sensors, configured to measure NOx levels in the exhaust system, may therefore be positioned in the exhaust system to detect failures of the NOx emission control system. Specifically, increases in NOx levels that may be indicative of degradation of one or more components of the NOx emission control system may be detected by the NOx sensors. Thus, the efficiency of the SCR catalyst and other components of a NOx emission control system may be monitored by one or more NOx sensors positioned in the exhaust system.
Current OBD (On-Board Diagnostics) regulations require the monitoring of exhaust NOx sensors to determine whether the NOx sensors have degraded (e.g., developed gain skew), as well as to determine whether the NOx sensors have developed an offset that may influence exhaust emissions. These two types of determinations are performed independently; typically, gain skew degradation is determined via a NOx sensor self-diagnostic (SD) test, whereas a separate test may be performed to determine whether the NOx sensor has developed an offset.
One approach for determining whether a NOx sensor has developed an offset includes performing a NOx offset diagnostic procedure during engine overrun (e.g., deceleration fuel-cut) conditions, in which engine combustion does not occur. This diagnostic procedure operates on the assumption that a properly functioning NOx sensor outputs a reading close to an ambient NOx value once the overrun conditions have continued for a long enough duration.
However, the inventors of the present application potential issues with the above solution. The SCR catalyst stores ammonia (NH3), and releases NH3 downstream when too much reductant has been injected or when temperature in the exhaust has increased to a certain extent. Once NH3 release from the SCR catalyst begins, it tends to release for a longer duration than the usual overrun duration. This is problematic as the NOx sensors currently on the market tend to confuse NH3 and NOx, and read NH3 as NOx. Thus, the output of a NOx sensor located downstream of an SCR catalyst may have an erroneously high NOx offset during release of NH3 from the SCR catalyst. The erroneously high NOx offset may cause the vehicle's Malfunction Indicator Light (MIL) to illuminate unnecessarily, resulting in warranty issues. In the future, as NOx emissions regulation becomes more stringent, NH3 release from SCR catalysts may occur even more frequently due to increased urea injection, thus undesirably increasing the likelihood of such warranty issues.
In one example, the issues described above may be addressed by a method which includes, during a soak period following vehicle-off of an engine-driven vehicle, waking up a powertrain control module and heating an exhaust NOx sensor. At light-off of the NOx sensor, the powertrain control module detects NOx sensor output, determines a duration to continue heating the NOx sensor based on the detected output, and continues to heat the NOx sensor until the duration ends. At the end of the duration, a NOx sensor offset test is performed. The inventors herein have recognized that by waiting for a duration (e.g., approximately 4 hours) after vehicle-off before waking up the powertrain control module, and then heating up the NOx sensor for an additional duration after it lights off before performing NOx sensor offset testing, encapsulated NOx, NH3, and moisture within the sensor protection tube may be dissipated, thereby avoiding an erroneously high NOx offset reading.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.